The correct answer is C) Double helical gears. Marine propulsion reduction gear sets typically use double helical gears because they provide several advantages over other gear types. Double helical gears have two opposing helical gear sections that mesh together, which helps to balance the axial thrust forces and provide a smoother, quieter operation compared to single spur or helical gears. This design is well-suited for the high-torque, high-power requirements of marine propulsion systems. The other answer choices are incorrect because single spur gears do not have the balanced thrust forces, single helical gears have unbalanced thrust forces, and double bevel gears are used for different applications, such as in marine stern drives, not in the main propulsion reduction gear sets.